Turbochargers are used to enhance performance of internal combustion engines that provide power to vehicles and other types of machinery. Turbochargers are typically centrifugal compressors driven by exhaust gas turbines that boost engine power, curtail emissions, and improve fuel economy. Rather than being belt-driven by the engine, as are superchargers, the typical turbocharger relies upon exhaust energy for its power, and as such is designed to rotate at speeds considerably higher than those of superchargers. Some turbochargers further include an electric machine, such as an electric motor or an electric generator, which drives and/or is driven by the exhaust gas turbine to assist the compressor during periods when insufficient exhaust energy is available to power the turbine or to generate additional electrical output utilized by the vehicle or machinery.
The challenges of satisfactorily lubricating and cooling turbochargers are well known by those skilled in the art. Turbochargers can rotate at speeds in excess of 100,000 rpm and therefore need lubrication for the rotating parts and cooling to remove heat generated by the rotating parts. Normally, different fluids are used to ensure adequate lubrication and cooling to the turbocharger. For example, turbochargers generally include a shaft configured to simultaneously drive or otherwise rotate the turbine wheel, compressor wheel, electric machine rotor and the like. The shaft includes a pair of spaced bearings located along its length, and the bearings are lubricated with oil or other such lubricant. Furthermore, the electric machine may be sensitive to prolonged exposure to temperatures of 200° C. and above and therefore requires cooling to maintain an acceptable operational temperature. As such, water or other coolant is circulated around the electric machine to conduct heat away from rotating components.
Use of multiple fluids for lubrication and cooling requires maintaining separate fluid sources and circulation systems. Such configuration increases turbocharger size and creates reliability and maintenance issues due to an increased number of fluid joints. As a result, modifications that allow a single fluid to be used for both lubrication and cooling will help with compliance to packaging requirements that continue to reduce physical spaces allotted to under-the-hood components including turbochargers. Additionally, elimination of separate fluids for lubrication and cooling will simplify the complexity of fluid circulation systems and assure continued reduction of turbocharger operational and maintenance costs.